Those genuinely wishing to donate money to our cause, we offer to ring straight back at our cost.
Very commendable, but if idb was interested in donating to your charity, then he would not be able to call from abroad in the first place, to enable you to call him straight back.
So there is a certain element of 'off-putting' involved, if you can believe that. We only rehome dogs to fully-checked and approved locations and individuals - we do not 'move dogs on' willy-nilly. It may sound strange to you, but after all no-one likes time-wasters.
How about offering 2 different additional numbers, 1 freephone, 1 geographic, for DONATIONS ONLY, or for VOLUNTEERS ONLY. Beside this, state "The freephone number is provided to allow you to call us free of charge. If you telecom provider allows free calls to geographic numbers, or you wish to use your mobile inclusive minutes please call 01....., or 02....... We have to pay a per minute charge for freephone numbers, and calling a geographic number will mean more of your donation will help us"
Keep the 0870, for people who as you mention are timewasters, and being able to be open and honest about the cost of the call, is certainly a step in the right direction.
On your website it might also be an idea to accept donations online. This would mean getting the payments would be all automated, and free up your telephone lines.
Also, perhaps you can accept enquiries online. If people want to volunteer, then you can call them back. If people want to become timewasters etc, then if they state the nature of their enquiry online, then you can reply saying "Thankyou for this enquiry, we can/cannot help you at this stage", or "Thankyou for this enquiry, to discuss the matter further, call 0870...."
If all of this were put in place, then if idb wanted to contribute, he would, either online, or you give him a callback abroad!! And most importantly, people who want to give you their money won't have to pay an extra charge via their phone company and be more likely to get on the "DOG AND BONE!!!" (Sorry I had to make this deliberate pun!!)